First commercially deployed in 1998, overlay network-based wireless location systems have been widely deployed in support of location-based services including emergency services location.
Iterative geolocation of a radio emitter through the use of TDOA may include the use of a single portable geolocation (e.g., TDOA) sensor, a pair of portable geolocation sensor stations and three of more portable geolocation sensor stations. Adding portable geolocation sensor stations to the iterative process reduces the constraints on the signals to be located as well as providing a reduction in the number of iterations required to obtain improved location accuracy.
Passively determining the location of a transmitter requires that characteristics of its signal be measured at a number of distinct, and known, locations by sensor stations. This requires that the locations of the sensor stations in the geolocation network be known (e.g., each sensor station knows its own location or a central controller knows the location of each sensor station) and are time synchronized to each other, i.e. they operate on a common time base. The required precision of this time base is dependent on the location technique used.
Additionally, the sensor stations should provide the transmitter's signal characteristics (e.g. timing, power, angle, signal quality) to a central node, via a communication network, where they can be processed, along with the locations of the sensor stations, to determine the transmitter's location.
The location of sensor stations can be determined by a number of means including manual surveys, use of an internal Global Navigation Satellite System (GNSS) timing receiver or reception of transmissions from stationary transmitters of known location (e.g., a commercial or governmental timing beacon or a time and frequency stable transmissions such as those from High-Definition Television (HDTV) transmitters). However, in some impaired environments (e.g., where one or more sensor stations are blocked or blind), none of these techniques is available for determining the location of a sensor. A wireless communication network is often used for communications between the sensor stations and this wireless communication network can serve a dual purpose for sensor stations that are not able to determine their location with other techniques. In this specification, the terms GNSS and Global Positioning System (GPS) will be used interchangeably.
The inventive techniques and concepts described herein apply to well known wireless systems, which include the widely deployed time and frequency division multiplexed (TDMA/FDMA) radio communications systems including the widely used Global System for Mobile communications (GSM) and OFDM-based wireless systems such as Long Term Evolution (LTE), WiMAN (IEEE-802.16) and WiMAX (IEEE-802.20), as well as code-division radio communications systems such as CDMA (IS-95, IS-2000) and Universal Mobile Telecommunications System (UMTS), the latter of which is also known as W-CDMA. Further background information may be obtained from U.S. Pat. No. 7,616,155, Nov. 10, 2009, “Portable, Iterative Geolocation of RF Emitters,” the disclosure of which is hereby incorporated by reference in its entirety.